Line-scan imagers can be used for a wide range of applications. These range from remote sensing from satellites or airplanes for precision agriculture, to imaging products on a conveyor belt for sorting or quality control, to imaging on a microscope for biomedical research. The imagery collected by a line-scan imager is used to identify an object, feature, or characteristic within the imaged region, which typically requires analysis of the image. Once the object, feature, or characteristic is determined, the information may be used, for example, to locate regions in need of fertilizer in crop fields, determine which items should be removed from a conveyor belt, or help determine biological response to a drug.
A line scan imager accepts light or other electromagnetic radiation and directs that light toward a detector. The detector typically includes a plurality of sensors arranged in a linear manner, where the detector produces an electrical signal based on the type and/or intensity of electromagnetic radiation that impinges on the sensors of the detector. A line scan imager is typically associated with images that correspond to long, thin rectangles, thus approximating a line. This image corresponds to the linear nature of the detector, so the line scan imager may be configured to “see” one long, thin rectangular region, typically called a “line,” across the width of the object imaged by the line-scan imager. The line scan imager can then take multiple readings as the object being imaged moves (or conversely, the line-scan imager moves), so each section of the region of interest is successively monitored by the line scan imager as the object of interest moves relative to the line-scan imager. Putting together the imaged “lines” sequentially provides an image of the region of interest. The line scan image, which typically includes a plurality of lines, can then be analyzed to identify objects, features, or characteristics within the region imaged by the line-scan imager. Light or other electromagnetic radiation sources may be directed at the region imaged by the line-scan imager, where the light may be reflected, transmitted, or emitted by items present in the imaged region. For example, one type of plastic may reflect a different intensity of electro-magnetic radiation at a particular frequency as compared to other types of plastic, such that the different types of plastics can be identified by the line scan imager. The information gathered by the line scan imager can then be transferred to an actuator that automatically sorts the different types of plastics or other items. Line scan imagers have many other uses, such as monitoring of geographic areas from a flying vehicle. Illumination may also be provided naturally, such as by the sun.
The line scan imager typically requires a certain magnitude of light or other signal to accurately detect and identify different items, similar to the human eye needing a certain amount of light to see and identify different items. The electrical signal produced by a sensor of the detector is generally proportional to several influencing factors, including: the radiance magnitude of the incoming light or electromagnetic radiation; the amount of time the light contacts the detector to produce the electrical signal; the sensor area; the efficiency of the sensor; and the spectral bandwidth of the incoming light incident on the sensor. Many line scan imagers include an aperture to limit the incoming light to a desired area, and the size of this aperture influences the electrical signal magnitude.
Often there is a desire to increase the line-scan frequency. For example, smaller objects may become of interest, thus requiring improved resolution, which in turn requires higher line-scan frequencies. Alternatively, there may be an economic incentive to increase the speed of a conveyor belt with items that are imaged by a line-scan imager, and thus the line-scan imager needs to image at a higher frequency. As the line-scan frequency increases, the amount of time for the sensor to produce an electrical signal decreases, and this lowers the electrical signal produced by the sensor. In some cases, the radiance can be increased by shining a brighter light or other electromagnetic source on the region of interest to increase the radiance. However, as the intensity of the electromagnetic radiation source increases, the amount of power required for that electromagnetic radiation source also increases. In some cases, the intensity of the electromagnetic radiation source is so high that the items of interest can be damaged, such as if the motion of a conveyor belt stops while the electromagnetic radiation source is operating. For example, the light source can be so intense it thermally damages (burns or cooks) the items on a conveyor belt if the conveyor belt stops, such as for mechanical reasons. Optical systems may be used to focus or otherwise manipulate the light or electromagnetic radiation before entry into the line scan imager, and these optical systems can improve operations in various manners. For example, a filter may remove light frequencies that interfere with the identification of certain items.
Accordingly, it is desirable to provide optical systems that improve the efficiency of line scan imagers. Furthermore, it is desirable to provide optical systems that will not degrade, or may even improve, the spatial resolution of line scan imagers. That is, it is desirable for line scan imagers to resolve smaller objects in their field of view than traditional line scan imagers, and an optical system that improves the efficiency of the line scan imager should not degrade its ability to resolve small objects. Other desirable features and characteristics of the present embodiment will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.